There is an increasing awareness of the health hazards presented by many common air pollutants. Perhaps in response to these concerns, governments are increasingly regulating the exhaust emissions of vehicles. In the United States, Environmental Protection Agency requirements relate to the exhaust of vehicles, rather than the device or method used to control the exhaust. Two predominant methods are currently used to control emissions; they are the utilization of alternative fuels, and solid particulate removal, as with a filter.
In particular, diesel engines, such as those utilized in trucks, buses, and passenger cars, produce a tremendous amount of soot. As there are in excess of 1.2 million diesel-powered vehicles in the United States alone, diesel engines pose significant health and air pollution problems. Over the next several years, vehicles powered by such diesel engines must meet more and more stringent regulations. As a result, there is increasing interest in the efficient and effective limitation of emission of particulate material, generally carbon and hydrocarbon particles, from the exhaust gases of diesel engines.
Various types of filtering devices have been proposed to filter diesel engine exhaust. Usually, such devices comprises filter systems which retain and collect the particulates in the exhaust gas. As soot particles are reported to range in size upward from 250 A (0.25 micron), a high efficiency filter is required to effectively filter out such contaminants. A number of filters are known. For example, cellular ceramic filters and honeycomb filters of porous ceramic material, such as those disclosed in U.S. Pat. Nos. 4,872,889 and 4,948,403 to Lepperhoff et al., have been recognized as being useful in trapping particulates from exhaust emissions.
However, particulates retained in the filter generally lead to an increase in the flow resistance in the exhaust and a resultant increase in the back pressure of the exhaust. Excessive back pressure can develop quickly, particularly when high efficiency filters are utilized. This unacceptable increase in exhaust back pressure can lead to an increase in fuel consumption, and, in extreme cases, to engine shut-off or failure. This result is particularly troublesome with truck and bus diesel engines inasmuch as any filter of a practical size would quickly become loaded and develop high back pressure which would result in engine shut-off.
As a result, it is necessary to intermittently regenerate the filter to remove the carbon particles from the filter during operation of the diesel engine. This is generally accomplished by providing sufficient heat to combust the particulates. Consequently, filter materials must withstand temperatures of over 600.degree. C. (1112.degree. F.) repeatedly. A number of methods of regeneration are known, such as the utilization of electric heating elements, as disclosed in, for example, U.S. Pat. No. 5,053,062 to Barris et al., U.S. Pat. No. 4,791,785 to Hudson et al., and U.S. Pat. Nos. 4,872,889 and 4,948,403 to Lepperhoff et al. Another means of regenerating the filter includes turbo enriched fuel injection to raise the temperature in the filter to initiate auto-combustion of trapped soot particles. These methods may suffer, however, from difficulties in the ignition of deeply trapped soot particles during regeneration or require an excessive energy input to regenerate the filter material.
Ceramic honeycomb filter designs are particularly susceptible to rapid development of excessive back pressure. There are a number of additional disadvantages, however, associated with the use of ceramic materials. Ceramic materials, particularly filters, are inherently brittle, and, consequently, subject to fracture from shock and mechanical stresses. Therefore, when ceramic materials are used in filters, it is necessary to design the filters with greater depth thickness than ordinarily desirable. Further, because ceramic materials are fragile and not deformable, it is not feasible to utilize standard engineering edge-sealing, gasketing methods due to the heating that is required. Ceramics are also costly to manufacture as they are difficult to shape. Additionally, inasmuch as the uniformity of ceramic particles is difficult to control, particularly for sintering and pre-forming, manufacturing quality is difficult to control.